Magnetometer



June 27, 1961 c, ARNOLD I 2,990,513

MAGNETOMETER Filed Sept. 2, 1958 2 Sheets-Sheet 1 INVENTOR.

JAMES C. ARNOLD June 27, 1961 J. c. ARNOLD 2,990,513

MAGNETOMETER Filed Sept. 2, 1958 2 Sheets-Sheet 2 AMPLIFIER Fig. 4

INDICATOR 46 5 INVENTOR-l' wlfi" 3 JAMES C. ARNOLD 1 62 m BY 4 6O 1 Fig.3

2,990,513 MAGNETOMETER James C. Arnold, 5771 Electric, La 10112:, Calif.Filed Sept. 2, 1958, Ser. No. 758,238 8 Claims. (Cl. 32443) The presentinvention relates generally to magnetic field detection apparatus andmore particularly to a magnetometer.

The primary object of this invention is to provide a magnetometer inwhich a novel type of cathode ray tube of simple construction is used asa signal producing element to operate a magnetic field indicator.

Another object of this invention is to provide a magnetometer in whichthe signal producing element is controlled by permeable bars which areoriented to detect a. magnetic field in a selected plane.

Still another object of this invention is to provide a magnetometer inwhich an initial variable magnetic field may be introduced adjacent thesignal producing element to adjust the instrument to a zero indicationas a starting reference.

A further object of this invention is to provide a magnetometer whichcan be completely enclosed in a compact, portable case to be carried inone hand, or mounted on a support such as a tripod.

Finally, it is an object to provide a magnetometer of the aforementionedcharacter which is simple and convenient to use and which will givegenerally efiicient and durable service with consistently accuratereadings.

With these and other objects definitely in view, this invention consistsin the novel construction, combination and arrangement of elements andportions, as will be hereinafter fully described in the specification,particu larly pointed out in the claims, and illustrated in the drawingswhich form a material part of this disclosure, and in which:

FIGURE 1 is a front elevation view of the magnetometer with the coverremoved;

FIGURE 2 is a fragmentary sectional view taken on the line 22 of FIGURE1;

FIGURE 3 is an enlarged fragmentary sectional view taken on the line 3-3of FIGURE 1; and

FIGURE 4 is a diagram showing the electrical circuitry of the completeinstrument.

Similar characters of reference indicate similar or identical elementsand portions throughout the specification and throughout the views ofthe drawings.

The magnetometer is based on a cathode ray type electronic tube in whichis a conventional electron gun having a heater 12 to energize a cathode14, the electrons emitted by the hot cathode being concentrated by agrid 16 and focused into a narrow beam through focussing anodes 18 and20, the beam being directed at a cap anode 22 at the end of the tuberemote from the cathode. The electron gun assembly is operated by asuitable power supply 24, the circuit involved being well known. Theelectron gun assembly is, of course, enclosed in an evacuated glassenvelope 26. Adjacent the cap anode 22 and spaced on opposite sidesthereof are two targets 28 and 30 which are connected to a diiferentialamplifier 32. Between the targets 28 and 30 and the anodes 20- are twogenerally rectangular deflecting plates 34- and 36 of magneticallypermeable material, said channel plates being opposed on opposite sidesof the tube axis and coplanar with said targets. The deflecting plates34 and 36 have opposed overlapping channel portions 38 and 40, definingbetween them a flat rectangular channel 42 elongated in the plane of thetargets 28 and 30, as in FIGURE 3.

The tube 10 is mounted in a socket 44 containing the necessaryelectrical connections, said socket being secured Patented June 27, 1961ice to a mounting plate 46 which is pivotally suspended from a gimbalassembly 48, said gimbal assembly being of any suitable design to enablethe mounting plate to hang plumb at all times. the partition 50 of acase 52 in which the complete magnetometer is contained, said casehaving a battery compartment 54 on one side of said partition togetherwith the amplifier 32 and power supply 24. On the other side of thepartition 50 is an open chamber 56 to allow freedom of movement for themounting plate 46. It should be understood that the case 52 andarrangement of its contents as illustrated are merely an example, thespecific configuration of the instrument as a unit being unimportant tothe present disclosure.

As illustrated, the tube 10 is disposed in a horizontal position on themounting plate 46 and is oriented so that the deflecting plates 34 and36 are parallel to the mounting plate in a vertical plane. Fixed to themounting plate 46 on opposite sides of the tube 10 are two verticallyopposed permeable bars 58 and 60, coplanar with the deflecting plates 34and 36 and having arcuate ends 62 to fit closely against said tube, asin FIGURE 3. The permeable bars 58 and 60 are mounted on suitablesupport blocks 64 in correct alignment, and fitted around said bars arebalancing coils 66 and 68, which are connected in parallel to a battery70, or similar source of DC. power through a switch 72. Connectedbetween the battery 70 and the coils 66 and 68 is a balancing control 74containing inversely proportionally variable resistors 76 and 78 havinga common tap 80, that is, any suitable arrangement wherein the currentin one balancing coil is increased while the current in the other coilis decreased.

In operation, the instrument is normally used to detect anomalies in themagnetic field of the earth for subsurface surveying, an anomaly usuallyindicating the presence of an ore body or other significant strata. Theinstrument is placed at a chosen location and the local magnetic field,or the Vertical component of the field induces a magnetic flux in thepermeable bars 58 and 60. The electron beam from the cathode 14 normallystrikes the cap anode 22, as indicated by the solid line directionalarrow in FIGURE 4. However, the magnetic field of the permeable bars5860 and the deflecting plates '34 and 36 deflects the electron beam,indicated by the dash line arrow. The electron beam, being confined inthe channel 42., is subject to deflection only in the plane of thetargets 28 and 30 and, when deflected, strikes one or the other of thetargets. After initial balancing, the degree and direction of deflectionof the electron beam are directly proportional to the intensity andpolarity of the local, natural magnetic field causing the deflection.Thus the energy imparted to the target bombarded by the beam is a directindication of the strength of the local, natural magnetic field, whilethe particular target bombarded denotes the field polarity. The signalenergy from the targets 28 and 30 is amplified by the amplifier 32 whichis arranged to differentiate between the signalsfrom both targets, thecircuitry of such an amplifier being well known. The output of theamplifier 32 is fed to a meter or indicator 82 which provides a visualindication of the magnetic field strength on a graduated scale, eitherpositive or negative according to polarity.

In recapitulation, since a point of reference is necessary to comparesuccessive readings, the instrument must be adjusted to a zero readinginitially. This is accomplished by means of the balancing control '74which controls the current flow in balancing coils 66 and 68, theinitial magnetic field induced in the permeable bars 58 and 60 causadeflection of the electron beam. By adjusting the balancing control 74,this initial field can be made to coun- The gimbal assembly 48 is fixedto teract the earths magnetic field at another location, by centeringthe electron beam and causing a zero reading on the indicator 82.Thereafter, the readings obtained at subsequent locationscan be comparedto the initial zero reading and the results plotted to indicate the.presence and extent of magnetic anomalies.

The instrument as illustrated is of simple form using a single tube withthe permeable bars 58 and. 60 disposed to detect a vertical magneticfield. However, by orienting the permeable bars to a horizontalposition, the horizontal field can be measured, the instrument beingrotated in the desired direction. Alternatively, three completeassembliesof tubes 10 with their respective permeable bars may be usedand aligned in three axes normal to each other to provide a total fieldreading, the amplifying means being modified to suit. It is obvious thatthis instrument can be used to delineate 4 stricted channelsubstantially limiting directional variation of the electrons tovariation in said plane; deflect ing elements disposed on opposite edgesof the electron beam path, said deflecting elements having their axes insaid plane and comprising a pair of permeable bars mounted on oppositesides of said tube, whereby a magnetic field induced in said permeablebars acts through 7 said channel portions to deflect the electron beamto one ,the targets corresponding to the polarity and intensity andmeasure magnetic fields Other than the earths magnetic field. The termdeflecting element employed in the original claims is proposed asincluding all such means ascoils, permeable bars or coils with permeablebars as cores.

In contrast to other types of magnetometersutilizing alternating currentof various frequencies with. the associated complex and bulky equipment,the present instrument uses a small variable direct current source andsimple circuitry. The electron beam is highly sensitive to deflectionand renders the instrument extremely sensitive to magnetic fieldvariations.

It is understood that minor variation from the form of the inventiondisclosed herein may be made without departure from the spirit and scopeof the invention, and that the specification and drawings are to beconsidered as merely illustrative rather than limiting.

I claim:

1. In a magnetometer: a source of electrons; a pair of spaced targetsdisposed essentially in one plane; means for directing a beam. ofelectrons from said source in a path normally extending between saidtargets; plates having channel portions restricting directionalvariation of said electrons in said path to variation in substantiallysaid plane only; deflecting elements disposed on opposite sides of theelectron beam path; said deflecting elements being capable of beinginfluenced by the local natural magnetic field to deflect said beamtoward one or other of said targets; and indicator means connected tosaid targets to indicate the reception of electrons by the targetscorresponding to changes in the polarity and intensity of the naturalmagnetic field as the magnetometer is travelled.

2. In a magnetometer: an electronic tube containing a source ofelectrons at one end thereof; a pair of spaced targets at the other endof said tube and disposed essentially in one plane; an anode disposedbetween said targets; means for directing a beam of electrons from. saidsource toward said anode; plates having channel portions between saidsource and said anode and defining a restricted channel substantiallylimiting directional variation of the electrons to variation in saidplane; deflecting elements disposed on opposite edges of the electronbeam path, said deflecting elements having their axes in said plane;said deflecting elements being capable of being influenced by the localmagnetic field to deflect said beamtoward one or other of said targets;and indicator means connected to said targets to indicate the receptionof electrons by the targets corresponding to changes in the polarity andintensity of the magnetic field.

3. In a magnetometer: an electronic tube containing a source ofelectrons at one end thereof; a pair of spaced targets at the other endof said tube and disposed essentially in one plane; an anode disposedbetween said targets; means for directing a beam of electrons from saidsource toward said anode; plates having channel portions between saidsource and said anode and defining a retit] of the magnetic field insaid permeable bars.

4. In a'magnetometerz an electronic tube containing a source ofelectrons at one end thereof; a pair of spaced targets at the other endof said tube and disposed essentially in one plane; an anode disposedbetween said targets; means for directing a beam of electrons from saidsource toward said anode; plates having channel portions between saidsource and said anode and defining a restricted channel substantially.limiting directional variation of the electrons to variation in saidplane; defleeting elements disposed on opposite edges of the electronbeam path, said deflecting elements having their axes in said plane; amounting plate; said tube being fixed on said mounting plate; saiddeflecting elements comprising a pair of permeable bars secured to saidmounting plate on opposite sides of said tube in said plane, whereby amagnetic field induced in said permeable bars acts through said channelportions to deflect the electron beam to one of said targets; saidmounting plate being freely pivotal'ly suspended and gravity controlledto maintain said permeable bars normally vertical; and indicator meansconnected to said targets to indicate the reception of electrons by thetargets corresponding to the polarity and intensity of the magneticfield in said permeable bars.

5. In a magnetometer: an electronic tube containing a source ofelectrons at one end thereof; a pair of spaced targets at the other endof said tube and disposed essentially in one plane; an anode disposedbetween said targets; means for directing a beam of electrons from saidsource toward said anode; plates having channel portions between saidsource and said anode and defining a restricted channel substantiallylimiting directional variation of the electrons to variation in saidplane; deflecting elements disposed on opposite edges of the electronbeam path, said deflecting elements having their axes in said plane andcomprising a pair of permeable bars mounted on opposite sides of saidtube with their axes in said plane, whereby a magnetic field induced insaid permeable bars acts through said channel portions to deflect theelectron beam to one or other of said targets; balancing coils wound oneach of said permeable bars; a source of DC. voltage connected to saidcoils; balancing means for varying the voltage in each of said coils ininverse proportion to the other; and indicator means connected to saidtargets to indicate the reception of electrons by the targetscorresponding to the polarity and intensity of the magnetic field insaid permeable bars.

6. In a magnetometer: an electronic tube containing a source ofelectrons at one end thereof; a pair of spaced targets at the other endof said tube and disposed essentially in one plane; an anode disposedbetween said targets; means for directing a beam of electrons from saidsource toward said anode; means for deflecting said electron beam, saidmeans comprising deflecting plates having overlapping oflset portionsdefining a flat, narrow width beam restricting channel therebetween,said channel being aligned in said plane and a pair of permeable barsmounted onopposite sides of said tube and having their axes in saidplane, whereby a magnetic field induced in said permeable bars actsthrough said channel portions to deflect the electron beam to one ofsaid targets; and indicator means connected to said targets to indicatethe reception of electrons by the targets corresponding to a thin, flatbeam restricting channel therebetween; a pair of permeable bars mountedon opposite sides of said tube and having their axes in said plane,whereby a magnetic field induced in said permeable bars acts throughsaid channel portions to deflect the electron beam to one of saidtargets; balancing coils wound oneach of said permeable bars; a sourceof D.C. voltage connected to said coils; means for varying the voltagein each of said coils in inverse proportion to the other; and indicatormeans connected to said targets to indicate the reception of electronsby the targets corresponding to the polarity and intensity of themagnetic field in said permeable bars.

8. In a magnetometer: an electronic tube containing a source ofelectrons at one end thereof; a pair of spaced targets at the other endof said tube and disposed essendaily in one plane; an anode disposedbetween said targets; means for directing a beam of electrons" from saidsource toward said anode; plates having channel portions disposed onopposite sides of the electron beam path and parallel to said plane;said channel portions defining a thin, flat beam restricting channeltherebetween; a mounting plate; said tube being fixed on said mountingplate; a pair of permeable bars secured to said mounting plate onopposite sides of said tube and having their axes in said plane, wherebya magnetic field induced in said permeable bars acts through saidchannel portions to deflect the electron beam to one of said targets;said mounting plates being freely pivotally suspended to maintain saidpermeable bars normally in a predetermined plane; balancing coils woundon each of said permeable bars; a source of D.C. voltage connected tosaid coils; means for varying the voltage in each of said coils ininverse proportion to the other; and indicator means connected to saidtargets to indicate the reception of electrons by the targetscorresponding to the polarity and intensity of the magnetic field insaid permeable bars.

References Cited in the file of this patent UNITED STATES PATENTS 252,025,580 Englehardt Dec. 24, 1935 2,437,374 Burroughs Mar. 9, 19482,702,882 Arnold Feb. 22, 1955

